Supplemental Report: Cracking and Leakage at ... - City of Pomona

ContentsIntroduction 1Inspection 2PageNovember 8 and 9, 2007 – Tank Roof 2Seepage 9December 5, 2007 – Tank Interior 9Petrography and Core Testing 16Discussion 17Concrete Cracking in Roof 17Interior Conditions 20Summary and Recommendations 21ii

Figure 1. Typical large cracking found at the Roof of Reservoir 5C.Figure 2. Typical roof cracking observed at Reservoir 5C.3

Figure 3. Overall view of roof showing scaling of the roof coating.To facilitate the inspection of the roof from underneath, the City of Pomona lowered the waterlevel in the tank such that an inspection could be performed from a small inflatable watercraftplaced into the water tank; a professional diver maneuvered the boat around to allow for ease ofinspection. Only a portion of the slab was able to be inspected in that manner, as the interiorwalls and baffles did not allow for access of the entire roof from below.Inspection of the roof from the interior revealed the following conditions:• The general condition of the roof was observed to be good with only minor crackingobserved (Figure 4 through 6). The crack widths were observed to be on the order of1/64 inch, with some locations showing signs of corrosion staining coming from thereinforcement (Figure 7).• The minor cracking ran along the length of the roof from east-to-west. Two distinctcracks were found approximately 8 feet and 4 feet from the inside face of the exteriorwall.• Larger cracks were found running north-to south, or perpendicular to the interiorlongitudinal wall, which corresponded closely with the large cracking discussed aboveand shown in Figure 1 and 2.4

• A total of 6 cores were taken from the roof structure, three of these cores were throughthickness of the suspended slab.• Each of the thru-thickness-cores was drilled directly above cracks running from northto-south(perpendicular to the longitudinal wall). Once the core was removed, the crackwas observed to extend the entire length of the core; i.e. the entire thickness of the slab;the crack was wider at the top than at the bottom (Figures 8 and 9).• Of the three cores drilled completely through the slab, two were found to beapproximately 9 inches long, whereas one of the cores was found to be approximately 8inches long, indicating the deck was approximately 8 inches thick in this location.• Concrete patch material was observed on the interior and exterior walls. The patchmaterial consisted of a thin crust-like, dark material, which was easily removed by handin some locations (Figure 10).Figure 4. Cracking at underside of roof is visible by observation of condensation.5

Figure 5. Cracking at underside of roof is visible by observation of condensation.Figure 6. Close-up of typical cracking running parallel to wall (moisture droplets had beenwiped down).6

Figure 7. Cracking and corrosion product under roof slab.Figure 8. Typical core removed from roof (Core #4 shown) illustrating crack along entire depthof core; the crack width is larger at the top of the core.7

Figure 9. Close-up of core #4.Figure 10. Spalling concrete patch material.8

SeepageACI 350.1 defines standard rates of seepage for hydrostatic tests of open or covered concretetanks. 1 Table 1 shows the designation and quantities for each of the different levels of loss. Atthe time of our inspection, a “snapshot” measurement of water drained through the tank seepagecollection system was approximately 520 gallons per day (gpd). It is important to note that thetotal amount of water leaving the tank may be underestimated through use of this test. A moreaccurate level of water tightness would be to very carefully observe the water levels in the tankrather than monitoring only the water being collected by the underdrain. Water loss may occurthrough the walls and into the ground, or directly into the ground through the slab-on-ground.ACI 350.1 contains appropriate measures to gauge water-tightness.For a 10,000,000 gallon tank, a seepage rate of 520 gpd represents 0.005% loss of water perday, i.e. five times less than the HST-025 designation. The construction drawings do notindicate water tightness requirements for this tank. The American Water Works Associationrecommends a maximum leakage allowance of 0.1 percent of the total volume over 24 hours,measured by a drop in the water surface not less than five days. 2Table 1. Designation and tightness criterion for concrete tanks. (Source: ACI 350.1)DesignationHST-NMLHST-025HST-050HST-075HST-100HST-VIOMaximum amount of waterloss to meet designationNo measurable loss0.025% per day0.050% per day0.075% per day0.100% per dayVisual inspection onlyDecember 5, 2007 – Tank InteriorThe City of Pomona had drained Reservoir 5C prior to the inspection by Dr. Piotr Moncarz, P.E.and Dr. Patxi Uriz, P.E. on December 5, 2007. A summary of findings is provided below:• Cracks were found at the slab on ground. A majority of these cracks had beenpreviously repaired by grinding and patching the crack with what appeared to be anelastomeric filler (Figure 11 and 12). A sketch of the crack pattern is shown in Figure13.1 “Tightness Testing of Environmental Engineering Concrete Structures and Commentary,” ACI 350.1-01,American Concrete Institute, 2001.2 “Summary Report on Concrete Water-Holding Structures,” AWWA, Committee on Concrete Water HoldingStructures, American Water Works Association Journal, Aug. 1978, Denver.9

• Some locations indicated cracks had been ground down, but no elastomeric materialwas used as a fill (Figure 14)• Some repairs did not continue to control joints, and in many of these cases the crackscontinued to extend beyond the repaired area showing signs of corrosion product.• Many un-repaired cracks were observed. Many of these cracks contained whatappeared to be a corrosion product (Figure 15).• Five cores were extracted from the ground slab (Core F1 thru F5). Cores F1 thru F4were cored directly over existing cracks. All cores were approximately 8 inches inlength, except for core F2 which was 11 inches long. Figure 16 shows the severity ofcracking as found in Core F3.• Cracks had been repaired also on the inside face of the exterior walls (FigureConsistently with the exterior observations reported in the initial report, and with thecalculations presented in that report, these cracks were more heavily concentrated at theground level and diminished in quantity along the height of the wall.• Concrete patch work was observed throughout the tank (Figure 19). The patch materialwas found to be placed on the walls vertically and around baffle openings. The patchmaterial in most observed locations was delaminating from the concrete wall, and waseasily removed by hand.• The concrete patch material near one of the baffle openings was delaminated, revealingits thickness and appeared as if a portion of the spall was missing (Figure 20).• A large spalled portion of concrete was found near one of the baffle walls whichappeared to have been from a section of wall just below the roof. No reinforcement wasfound in this spalled portion of concrete however, the location from which it had spalledappeared to have exposed reinforcement.10

Figure 11. Repaired concrete cracks.Figure 12. Close-up of concrete crack repair.11

Figure 13. Typical crack pattern for portion of tank floor (not to scale).Figure 14. Previously ground cracks, with no filler present.12

Figure 15. Core F1, illustrating length and severity of cracking in slabFigure 16. Cracking and corrosion product near location of Core F3.13

Figure 17. Repaired cracking on inside face of exterior wall.Concrete patchmaterialFigure 18. Repaired cracking on inside face of exterior wall along with concrete patch material.14

Figure 19. Close-up of delaminating concrete patch material.Figure 20. Delaminating concrete patch on baffle opening.15

Petrography and Core TestingOne of the cores (Core #6) from the roof was examined through petrographic examination, 3 andtwo cores (Core #1 and #3) were crushed to determine compressive strength. 4 Relevant findingsfrom these tests are reported below:• The concrete was properly consolidated and cured as determined by petrographicanalysis.• The Portland cement was sufficiently hydrated, and the water to cement ratio appearedto be low.• The longitudinal crack through the length of the specimen contained secondary calciumcarbonate deposits.• The crack contained fractured aggregate particles, indicating the concrete had achievedsufficient strength prior to the formation of the crack.• Compressive strength tests for Core #1 and Core #3 were 6,210psi and 6,590 psi,respectively.3 Petrographic Examination of Concrete Core Specimen, Job Number C-4821 A-07, December 19, 2007, Micro-Chem Laboratories, Murphys, California.4 Laboratory Test Results, DCI No. 6817-M04, December 3, 2007, Dynamic Consultants, Inc, Mountain View,California.16

DiscussionConcrete Cracking in RoofShrinkage of concrete is a time-dependent decrease in volume compared to its originalplacement volume (see also the February 16, 2007 initial report). This well-known phenomenonis an integral part of any reinforced concrete design, as constrained concrete shrinkage can leadto tensile cracking of concrete. If the cracks are large enough, they prevent the high-alkalinitycement paste from protecting the embedded steel reinforcement (rebar) from corrosion.To accommodate shrinkage and temperature dimensional changes of the large roof area,contraction joints in the roof are called out on structural drawings to be approximately 30 feeton center. The joints are designed to alleviate stress buildup in concrete and control the locationof cracking by purposely creating a weak plane in the cross-section of the slab. At the tank’sroof, the week plane was created by a saw cut soon after placement, or a tooled line in theconcrete where a crack would be expected to form. The American Concrete Institute (ACI)defines two common types of contraction joints, full and partial. 5 In full contraction joints, allof the reinforcement perpendicular to the joint is terminated at the joint (see Figure 21),additional smooth dowels are added (if needed) to help transfer shear loads across the crack. Inpartial contraction joints, at least 50% of the reinforcement is terminated at the joint, which mayalso utilize smooth dowels to transfer shear (if needed. Continuous reinforcement is helpful forregions of high seismic hazard (such as Pomona) to transfer tensile loads across the joint. Inboth cases, waterstops are typically placed across the joint to protect against rain water leakage.The structural drawings for the tank indicate all reinforcement to be continuous through thecontraction joint (see Figure 22); in essence this creates a partial contraction joint. A waterstopis called out to be located at the mid-depth of the slab in between the top and bottom layers ofreinforcement. A crystalline membrane has been specified on the roof structure to provideadditional sealing surface against rain water penetration. As the crystalline membrane is likelyto have crack over the contraction joints, water intrusion may result in reinforcement corrosionfor all reinforcement located above the waterstop.Table 2 gives recommended minimum values for temperature and shrinkage reinforcement forvarying distances between contraction joints. For contraction joints located 20 feet to 30 feetACI 350-06 recommends a reinforcement ratio (ratio of area of concrete to the area of steel in agiven cross-section, ρ) of 0.003 for strength of reinforcement specified (60 ksi). 6 For partial5 “Design Considerations for Environmental Engineering Concrete Structures,” ACI 350.4R-04, AmericanConcrete Institute, 2004.6 “Code requirements for Environmental Engineering Concrete Structures and Commentary,” ACI 350-06,American Concrete Institute, 2006.17

contraction joints, it is recommended that this number be multiplied by 1.5, which would elevatethe recommended reinforcement ratio to 0.0045. The reinforcement ratio for shrinkageprovided in the structural drawings is 0.0036 (#4 at 12 inches on center – each face).Furthermore, ACI 350-06 section 7.12.1.2 also stipulates that for regions of high constraint(where concrete is restricted from free shrinking), special considerations need to be given for theincreased amount of temperature and shrinkage reinforcement required. The roof slab atReservoir 5C is cast monolithically with the interior walls and large beams in each of the fourroof quadrants. This results in a restraint to the slab when trying to shorten in the longitudinaldirection.Reinforcing steel is placed inside the concrete elements to provide additional strength incarrying tensile stresses. Once the concrete cracks perpendicularly to the rebar, the rebar plays aprincipal role in keeping the crack width under the control. If insufficient reinforcement isprovided, it becomes ineffective in controlling the distribution and the width of cracks, resultingin fewer, wider cracks. As shown in our previous report ACI 224 7 reports reasonable crackwidths for the roof would be about 1/64 inch (0.016 inch) (Table 3). The larger, more widelyspaced cracks observed were more than twice that size. Repetitive loading (such as temperaturecycles) of narrow cracking has been observed to over the time double the widths of cracks. ThePomona tank can be considered a relatively new structure thus, in the long term, it is to beexpected that the observed cracking will continue to expand.Figure 21. ACI 350.4 recommended contraction joint details.7 “Control of Cracking in Concrete Structures,” ACI 224R-01, American Concrete Institute, 2001.18

Figure 22. Contraction (construction) joint from structural drawings. Note: reinforcement iscontinuous through contraction joint. (Source: Sheet DS-2 of the constructiondrawings)Table 2. Minimum shrinkage and temperature reinforcement. (Source: ACI 350-06)19

Table 3. Guide to reasonable* crack widths, reinforced concrete under service loads. (Source:ACI 224)Interior ConditionsThe general conditions of the interior concrete in the tank were good. No indications of visibledeterioration or construction defects which would lead to immediate serviceability concernswere identified. The repaired cracks in the slab-on-ground were found to be generally in goodcondition. In locations where new (possibly existing prior to previous repair) cracks wereobserved, corrosion product was observed at the surface, and extraction of cores illustratedfairly wide cracking. Some of those cracks previously not existing or not repaired arecandidates for repair in one of the soonest tank emptying cycles.The corrosion product observed is a potential problem for long-term serviceability. The loss ofreinforcement steel can potentially weaken the concrete slab leading to reduced water-tightnessduring the intended life of the tank. For this reason, the observed cracking with corrosiondeposits should be repaired to maintain serviceability.Similar to the cracking in the slab-on-ground, the observed repairs on the wall did not showsigns of deterioration. The repaired wall cracks appeared to be fairly stable, and while leakingto the exterior notwithstanding (see the initial Exponent report), these walls were in goodcondition.Cementitious patching repairs to the concrete walls were delaminated in many cases. In thedelimitation observed near the baffle opening, the very thick patch material was no longeradhered to the wall, rendering it ineffective. It is important to maintain the repairs to the levelwhich was intended during its original design or in the post-construction problem elimination.20

Summary and RecommendationsBased on the inspections and analysis the concrete tank structure is in a good condition.Although the roof structure has some large cracks, they do not seem to constitute any importantchange in the operability nor in the structural capacity of the structure. The general concretequality was very good, as verified by petrographic examination and visual inspection. Repairsmade previously to the slab-on-ground are in good condition and appear to be working properly.The amount of seepage observed through the underdrain did not raise a serious concern forsafety or large amounts of water loss – although a more rigorous test would be recommended tobenchmark, and accurately gauge, the total amount of water leakage from the tank over thefuture life of the tank..In order to maintain the structure in good working condition for the rest of its intended operatinglife, Exponent recommends the following:• Monitor external wall seepage and for clear water flow on the surface carry out interiorinspection of the area and repair the corresponding interior cracks(s) at the nextmaintenance inspection of the emptied tank. Should the seepage become visiblesignificant and the next scheduled maintenance more than a few months away, considerthe condition as requiring exception attention.• Repair the large cracks on the roof greater than 0.012 inch wide by grinding and fillingcracks with an elastomeric material suitable for exposed exterior conditions.• Continue inspection and maintenance on existing crack repair to the slab-on-ground andwalls on the tank interior.• Grind and repair other cracks observed at the slab-on-ground, especially those withcorrosion deposits visible.• The concrete patch material on the inside of the tank needs to be repaired.• A more rigorous test of the seepage rate is recommended to determine if water seepageis excessive, and determine the location of leakage if the amount is determined to beexcessive.21